proceeds, the mother liquor will become enriched in bdc-X,
which contributes to the growing proportion of bdc-X
incorporated.
In conclusion, we have demonstrated that the preferential
incorporatation of bdc over bdc-I and bdc-Br occurs in MOFs
of the general formula Zn4OL3, and that this selectivity is
related to the relative rates of crystallisation. An important
implication of these results is that the composition of MC-MOFs
is not necessarily uniform. In the system under study here, at
least, the proportion of linkers that bear a halo group is lower
towards the centre of the crystals than it is on the outside.
This anisotropy will have an impact on the properties of these
MC-MOFs, including post-synthetic modifications.
This work shows that the formation of MC-MOFs through
control of the incorporation of ligands that have the same
structural role provides opportunities to tailor the composi-
tion and function of MOFs. Current work is engaged in
expanding the scope of MC-MOFs and assessing the generality
of the results reported in this work.
Fig. 4 The relationship between the percentage of bdc-X incorporated
into [Zn4O(bdc)3ꢀx(bdc-X)x] and the reaction time. For , X = I, and
for , X = Br.
expected to be lower than those for H2bdc (pKa1 3.51, pKa2
4.82) given the inductive effect of the halo group. A similar
effect is notable for the benzoic acids: compare benzoic acid
(pKa 4.20) with 2-iodobenzoic acid (pKa 2.85) and 3-iodobenzoic
acid (pKa 3.86). This means that the concentration of the
2-halo-1,4-benzenedicarboxylates (and the [Hbdc-X]ꢀ mono-
anions) would be higher than that of bdc (and [Hbdc]ꢀ). Given
the greater inclusion of bdc over bdc-I and bdc-Br, the relative
concentrations of the ions cannot be the major factor in their
incorporation into the MOF crystals. However, the greater
basicity of the anions derived from H2bdc would be expected
to make these anions better ligands, so it is conceivable that
the greater ligating ability of the bdc ions wins out over the
lower concentration.
The EPSRC are thanked for financial support.
Notes and references
1 J. L. C. Rowsell and O. M. Yaghi, Microporous Mesoporous
Mater., 2004, 73, 3.
2 G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
´
3 R. Robson, Dalton Trans., 2008, 5113.
4 S. Horike, S. Shimomura and S. Kitagawa, Nat. Chem., 2009, 1,
695.
5 A. U. Czaja, N. Trukhan and U. Muller, Chem. Soc. Rev., 2009,
¨
38, 1284.
6 L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38,
1294.
7 J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38,
1477.
Observations on the crystal growth rates of [Zn4O(bdc)3]
(MOF-5), [Zn4O(bdc-Br)3] (IRMOF-2) and [Zn4O(bdc-I)3]
suggested another factor to be relevant too. When reactions
to form [Zn4O(bdc)3], [Zn4O(bdc-Br)3] and [Zn4O(bdc-I)3]
were set up side by side using identical conditions, crystals
were always observed first for [Zn4O(bdc)3], with those of the
the halo-containing products taking longer to form. This
suggested that the faster crystal growth rate for [Zn4O(bdc)3]
might be a factor in the observed greater proportion of bdc in
the mixed bdc/bdc-Br and bdc/bdc-I products.
8 L. Ma, C. Abney and W. Lin, Chem. Soc. Rev., 2009, 38, 1248.
9 D. N. Dybtsev, H. Chun and K. Kim, Angew. Chem., Int. Ed.,
2004, 43, 5033.
10 A. D. Burrows, CrystEngComm, 2011, DOI: 10.1039/
C0CE00568A.
11 K. Koh, A. G. Wong-Foy and A. J. Matzger, Chem. Commun.,
2009, 6162.
12 S. Furukawa, K. Hirai, K. Nakagawa, Y. Takashima, R. Matsuda,
T. Tsuruoka, M. Kondo, R. Haruki, D. Tanaka, H. Sakamoto,
S. Shimomura, O. Sakata and S. Kitagawa, Angew. Chem., Int.
Ed., 2009, 48, 1766.
13 Z. Wang and S. M. Cohen, Chem. Soc. Rev., 2009, 38, 1315.
14 S. M. Cohen, Chem. Sci., 2010, 1, 32.
In order to assess this, the reactions between zinc(II) and
50 : 50 mixtures of H2bdc and H2bdc-X (X = Br, I) were
carried out for shorter periods of time, with the products
analysed after 48 h, 72 h, 96 h, 120 h and 168 h. As expected,
shorter reaction times led to lower yields, and indeed no
product was observed for the reaction of zinc(II) with
H2bdc/H2bdc-Br after 48 h. In all other cases, however, there
was enough material to digest and analyse by 1H NMR
spectroscopy. These results are summarised graphically in
Fig. 4. With shorter reaction times there is clearly a lower
incorporation of bdc-Br or bdc-I into the MOF material, with
the degree of incorporation going up with increasing reaction
time. This is consistent with greater preferential inclusion of
bdc at the early stages of crystal growth. As the reaction
15 H. Chun, D. N. Dybtsev, H. Kim and K. Kim, Chem.–Eur. J.,
2005, 11, 3521.
16 H. Deng, C. J. Doonan, H. Furukawa, R. B. Ferreira, J. Towne,
C. B. Knobler, B. Wang and O. M. Yaghi, Science, 2010, 327, 846.
17 W. Kleist, F. Jutz, M. Maciejewski and A. Baiker, Eur. J. Inorg.
Chem., 2009, 3552.
18 A. D. Burrows, C. G. Frost, M. F. Mahon and C. Richardson,
Angew. Chem., Int. Ed., 2008, 47, 8482.
19 S. Marx, W. Kleist, J. Huang, M. Maciejewski and A. Baiker,
Dalton Trans., 2010, 39, 3795.
20 K. M. L. Taylor-Pashow, J. Della Rocca, Z. Xie, S. Tran and
W. Lin, J. Am. Chem. Soc., 2009, 131, 14261.
21 T. Fukushima, S. Horike, Y. Inubushi, K. Nakagawa, Y. Kubota,
M. Takata and S. Kitagawa, Angew. Chem., Int. Ed., 2010, 49,
4820.
22 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe
and O. M. Yaghi, Science, 2002, 295, 469.
c
3382 Chem. Commun., 2011, 47, 3380–3382
This journal is The Royal Society of Chemistry 2011